This invention relates to a method for manufacturing an array substrate; and in particular to a method for manufacturing an array substrate of an active matrix type liquid crystal display device where a switching device formed of a thin film transistor (TFT) is employed.
Since a liquid crystal display device has prominent advantages in the aspects that it is not bulky, is light in weight and is low in power consumption, it has been extensively utilized in a display device of office automation (OA) apparatus such as a word processor and a disk-top type personal computer in recent years. In conformity with this trend, the development of liquid crystal display device employing a thin film transistor or a thin film transistor array where a polycrystalline silicon is employed as a material constituting an active layer is actively undertaken with a view to improve the performance of the display device.
A thin film transistor where polycrystalline silicon is employed for an active layer (i.e. a polycrystalline silicon thin film transistor) has been conventionally employed in a switching device of pixel portion constituting a display portion of liquid crystal display device, or in a driving circuit of a pixel switching device. Namely, the polycrystalline silicon thin film transistor is utilized as a pixel thin film transistor for applying a voltage to a liquid crystal device, or as a driving circuit thin film transistor for driving this pixel thin film transistor.
However, since this polycrystalline silicon thin film transistor was subjected to a heat-treatment of as high as 800 to 1100.degree. C. in the manufacturing process thereof, a substrate made of a heat resistive material such as quartz was required to be employed for this polycrystalline silicon thin film transistor.
However, owing to the recent advancement in processing technique, the processing temperature of the polycrystalline silicon thin film transistor is now lowered so that it has become possible to form the polycrystalline silicon thin film transistor on an insulating glass substrate. However, as compared with the conventional manufacturing process of a transistor formed of amorphous silicon, a higher processing temperature is still required in the heat treatment of the polycrystalline silicon thin film transistor.
For example, a temperature of about 400 to 600.degree. C. is still required in a dehydrogenation heat treatment as a pre treatment to obtain a polycrystalline silicon using a laser, in a heat treatment for stabilizing the interfacial characteristic between a gate oxide film and a polycrystalline silicon film, or in a heat treatment for lowering the resistance of source/drain.
The properties of polysilicon thin film transistor is much influenced by the conditions of these heat treatments.
When an insulating glass substrate is heat-treated at a temperature of about 400 to 600.degree. C., the glass is shrunk thereby giving rise to the problem of deviation in pattern during the PEP process. For example, it has been found according to experiments made by the present inventors that when an insulating glass substrate (for example, NA35, a trade name of NH Techno Co. or #1737, a trade name of Corning Co.) is heat-treated at a temperature of 600.degree. C. for 5 hours and then left stand to cool, a shrinkage of about 11 ppm was admitted.
If a shrinkage of substrate is generated to such an extent, a deviation will be generated between the central portion and the peripheral portion of the substrate after the heat-treatment thereof.